Homogeneous catalysis plays an important role in the discovery of new materials such as polymers from the polymerization or copolymerization of olefins and often involves the use of organometallic catalysts. Typically, the active organometallic catalyst is generated by treatment of a catalyst precursor with a chemical component such as a suitable activator and/or scavenger. For olefin polymerization it is often preferable to generate the active catalyst in the presence of at least one of the olefinic monomers to be polymerized and under conditions where the concentrations and ratios of the monomers being polymerized are carefully controlled.
This is partly because some catalysts will decompose more readily in the absence of the monomer(s) to be polymerized (they are, in effect, stabilized by the presence of the monomers). It is also often important to generate active catalysts in the presence of all the monomers being polymerized and under conditions where the monomer concentrations and ratios are carefully controlled because the composition, structure or properties of the polymer being produced may be adversely affected otherwise. This may be challenging for gaseous monomers such as ethylene, propylene and isobutylene where the generation of an active catalyst is generally preferably done under pressure equilibrated conditions.
The application of combinatorial methodologies to the discovery of new materials such as polymers continues to receive considerable attention in academia and industry because it has the potential to increase greatly the rate of discovery over conventional discovery methods. U.S. Pat. No. 6,030,917, incorporated herein by reference, owned by the Assignee of the present application and entitled Combinatorial Synthesis and Analysis of Organometallic Compounds and Catalysts, issued February 2000 to Weinberg et al., discusses general combinatorial methods for preparing organometallic compounds such as catalysts. And PCT Application No. PCT/US00/00418, incorporated herein by reference, published July 2000 and owned by the Assignee of the present invention, discusses library formats for ligand arrays that may be used in the application of combinatorial methodologies.
A typical workflow that utilizes combinatorial methodologies for the discovery of new catalysts such as homogeneous catalysts that may polymerize olefinic monomers involves the screening large arrays of potential catalysts for their activity. The most active catalysts may be later screened under more carefully controlled conditions to ascertain polymer properties, composition or structure.
One such method for simultaneously determining the activity of large libraries of catalysts involves the use of infrared thermography (“IRT”) where a digital infrared camera is used to image (e.g., digitally photograph) the library of catalysts in the presence of one or more polymerizable monomers. Active catalysts are easily discerned from less active or inactive ones by comparing integrated temperature changes produced over time for each catalyst. See, for example, U.S. Pat. No. 6,063,633 to Wilson, entitled “Catalyst Testing Process and Apparatus,” incorporated herein by reference, discussing a method and apparatus of testing a plurality of catalyst formations to determine the comparative catalytic activity of the formations in the presence of a given reactant or reactant mixture.
In a typical experiment using IRT, an array of potential catalyst precursors in a standard 96-well microtiter plate may be treated with the desired amounts of one or more polymerizable monomers, activators, scavengers or other agents that may generate or stabilize an active catalyst, and the activity of the catalysts in the array is then monitored. For experiments that involve the use of gaseous monomers such as ethylene, propylene, or isobutylene, an array of catalyst precursors is typically first treated (if necessary) with liquid co-monomers and then pressurized in a high pressure reaction chamber for a length of time suitable to allow dissolution of the gaseous monomer(s) into the individual solutions of the catalysts precursors. The reaction chamber is then typically depressurized, to allow for and activators and/or scavengers to be added to the reaction chamber. The library is returned to the reaction chamber, and the reaction chamber repressurized with the gaseous monomer to run the reaction of interest. Catalyst activity is then monitored by IRT through an infrared (“IR”) transparent window mounted to the reaction chamber so as to be situated above the library and in optical contact with an infrared camera (“IR camera”).
This process has been successfully used to identify active catalysts in a single 96-element array format but may be problematic in several respects. For example, the step of pressurizing the array just prior to commencing data acquisition with gaseous monomers such as ethylene may introduce unwanted thermal signatures in the data being collected. This is because during pressurization the heat of compression of the gas causes a slight increase in the local temperature of the reaction chamber and therefore the catalyst solutions in the array. When the gas has reached a final pressure and there are no pressure-volume changes or other adjustments experienced by the system, the wells in the library begin to cool down towards ambient temperature. This cooling exotherm is then superposed over exotherms generated in the system due to catalyst activity in the wells and has to be subtracted from the data gathered in order to accurately extract the heat produced by these catalytic events. This can be particularly problematic for monitoring catalysts that are not very active or where the cooling exotherm is of the same or greater scale than that of the exotherms generated due to catalysis. The cooling exotherm may also interfere with efforts to obtain reliable data early in an experiment or for catalysts that are most active early on where the thermal noise due to the cooling exotherm is greatest.
In addition, if the reaction chamber is depressurized (e.g. to allow the library to be removed so that activators may be added), the gaseous monomers begin to outgas from the solutions in the wells of the library. As a result, the gaseous monomers are no longer present at the same concentration that they were when under pressure. For polymerization reactions involving more than one olefinic monomer, this may mean that the monomer ratios are different at the time of activation. This event creates a less than ideal situation because the monomer ratio and concentration may be critical to catalyst performance as well as polymer composition, property and structure—especially for highly active catalyst systems. Moreover, the workflow described above may not be readily adapted to allow screening of more than one library at a time. This is because the aforementioned outgassing effects would be significantly exacerbated in the time between activation of successive libraries.
Therefore, it would be desirable to generate the active catalyst in-situ, and preferably, after pressure and (subsequently) temperature equilibration have been reached without the need to depressurize the reaction chamber in order to remove and activate the catalyst precursor libraries. Additionally, it would also be desirable to increase the overall experimental throughput capacity of screening systems to permit screening of multiple samples or libraries in parallel or using serial methods.